The invention relates generally to weld wires or electrodes useful in joint and surface welding and more specifically to flux cored weld wires.
Flux cored weld wires are commonly employed in electric arc welding of mild and low alloy steel base metals. Flux cored wires are used increasingly as an alternative to solid weld wires for improved productivity in structural fabrication. These wires yield high strength welds in single pass and multiple pass welding operations at high welding speed. Flux core weld wires are composite tubular filler metal electrodes having a metal sheath and a core containing a composition of various powdered materials. The core composition comprises approximately 1 to 45% of the total wire weight. During the manufacture of the wire, the core composition is blended and deposited onto a steel strip, which is formed into a tube or sheath about the core composition in a forming mill. The steel sheath surrounding the core composition is then drawn or rolled through reducing dies to a specified diameter.
Manganese is an essential alloying addition in most welding electrodes. Manganese has several functions. It is a key strengthening alloy to the weld deposit; it chemically reacts with sulphur components in the molten weld metal, it acts as a de-oxidizer, and it affects weld puddle control, wetting action and general ease of use of the welding electrode itself. The current levels of manganese used in conventional electrodes reflect the optimum combination of alloying elements with iron to produce grades of welds that meet standard levels for strength and ductility.
Recently, the amount of manganese present in the fume generated when welding with electrodes containing manganese has been scrutinized for possible health or safety related issues. Governmental authorities have considered regulations to limit the amount of manganese. However, it is not feasible to eliminate manganese in conventional welds without negatively affecting the mechanical properties of the weld. The use of other alloying elements has been considered in an effort to lower total manganese levels while maintaining sufficient mechanical property levels in the weld without success. To date it has not been possible to significantly reduce the use of manganese in conventional weld wires and electrodes.
It has been found that the amount of manganese that is present in the fume that is generated during arc welding can be reduced significantly if the manganese is incorporated into the electrode as a composite particle that contains a shielding material. As the electrode melts during the welding operation, the shielding material is believed to prevent the manganese from oxidizing. As a result, more manganese resides in the weld deposit and less is present in the fume. The preferred shielding material is TiO2 and more particularly rutile TiO2.
Accordingly, one manifestation of the invention is a flux cored welding electrode in which manganese is incorporated entirely or partially into the electrode fill as a composite particle of manganese and shielding material.
Another manifestation of the invention is a process for arc welding using the aforementioned electrode. This process is advantageous because the fume contains significantly reduced levels of manganese.
Another manifestation of the invention is a process for arc welding on low carbon steel substrate using an electrode including a core fill containing manganese wherein the fume contains less than 7% by weight of manganese.
Another manifestation of the invention is a composite particle of manganese and shielding material that is useful in the electrode fill of a flux core welding electrode.